Modern hair dyeing methodology has developed from its initiation in the 1950's to the point where, today it is the third largest product type in the hair care category following shampoos and conditioners.
A wide variety of hair dyes or colorants have been developed by the art. These include direct dyes, autoxidative dyes, oxidative dyes and dyes which achieve their desired effect by conversion to a melanin. These latter dyes include dihydroxyphenylalanine (DOPA) 5,6-dihydroxyindole (DHI) and related compounds such as 5,6-dihydroxyindole carboxylic acid (DHICA) and its lower alkyl esters as well as N-lower alkyl derivatives of these compounds and acyloxy analogs such as 5,6-diacetoxyindole (DAI).
The most commonly used method of dyeing hair, particularly human hair, is oxidative dyeing in which a mixture of aromatic compounds, generally of the benzenoid series, containing a plurality of amino and hydroxy functions, which are themselves colorless, are converted by coupling reactions and oxidative processes, both known to those skilled in the art, to a blend of colored compounds within the hair fibers. The colorless aromatic compounds, in a suitable base formulation, normally are mixed with hydrogen peroxide or other strong oxidizing agent shortly before use. The colored compounds or dyes, typically, are formed by oxidative coupling between primary intermediates (usually diaminobenzenes or aminophenols) and couplers which are phenols or related cyclic compounds. Different shades are developed by using a mixture containing more than one of both the intermediate and the coupler.
It is also known to dye hair with certain compounds which, while they are useful as primary intermediates in association with couplers, may also be used alone since they may be directly oxidized either with an oxidizing agent or, with certain of the more sensitive compounds, by exposure to air. The oxidized primary intermediate can couple with unoxidized primary intermediate to form a dye.
Another recently developed method for dyeing hair involves formation of melanin-like pigments by oxidative conversion of appropriate precursors. The fundamental chemistry underlying this innovative technology is discussed in detail by G. Prota in Melanins and Melanogenesis, Academic Press, San Diego (1992). The principal melanin precursors employed by the art are DHI and its analogs including, for example, DHICA and its lower alkyl and alkaryl esters such as methyl, ethyl and benzyl esters and DAI. These, and other DHI analogs are hair dye precursors which may be oxidized to melanin under known conditions.
All of the foregoing compounds may be considered as oxidative hair dye precursors, in that under appropriate oxidizing conditions they will oxidize to form hair dyes or colorants.
It is well known that oxidative dyeing of hair can be achieved by first mixing a solution of dye precursors with a solution of oxidant (usually hydrogen peroxide or a salt which forms hydrogen peroxide). The final mixture is then applied directly to hair fibers. While this is the standard procedure for dyeing hair suggested by existing products in the market, it is not without disadvantages. The major concern is the destructive damage of hair during the dyeing process.
To circumvent this shortcoming, German DE 2,028,818 suggests metal pretreatment of hair. This enables the user to shorten the dyeing time. As a result, damage to hair may be minimized. However, such treatment is insufficient for total elimination of damage to hair induced by peroxide.
U.S. Pat. No. 5,100,436 describes the use of 2,2'-dipyridyl and o-phenanthroline metal ion chelates in hair dye mixtures utilizing hydrogen peroxide or oxidizing agents which function by the production of hydrogen peroxide.
In U.S. Pat. No. 5,032,138, use of a chlorite in lieu of hydrogen peroxide is suggested. The process significantly reduces oxidative damage of hair. The draw backs of this method are that the color of the treated hair tends to be weak, a long period of time is thus necessary to produce a desired color intensity, and a large molar excess of chlorite is required.
The art, therefore, is constantly seeking more convenient dyeing methods which can be used with all types of oxidative dye precursors without damaging the hair. According to the present invention, it has been found that certain redox metal salts such as copper, iron, manganese, or cobalt salts can be used in combination with chlorite to dye hair. This, on the one hand, avoids the oxidative damage of hair, and, on the other hand, affords more intense and rapid coloring of hair than if chlorite is used alone.